In the field of electronic signal processing, it is frequently necessary to accurately balance (align) frequencies or frequency-dependent circuits, for example, when adjusting tuners or balancing frequency generators.
Corresponding AFT (automatic fine tune) techniques are known from, for example, "Television Engineering Handbook", page 13.117, K. Blair Benson, McGraw-Hill Book Company. In this the phase shift in a parallel resonant circuit is evaluated.
FIG. 1 shows an appropriate block circuit diagram. An output signal 10 influenced by a tuning, for example, a television audio signal, is fed to a resonant circuit 11 and a first input of a phase detector 12. The resonant circuit is tuned to a desired frequency. The output signal from the resonant circuit 11 is fed to a second input of the phase detector. The output signal from the phase detector is filtered in a low-pass 13 and serves as regulating variable 14 for the tuning of the input signal 10. A circuit suitable for this is, for example, an FM demodulator 15 which can be tuned by a regulating variable and to which the regulating variable 14 and a signal 16 for demodulating are fed.
In this particular application, however, only a tuning accuracy of about 150 kHz can be achieved with the state of the art. Apart from that, such known AFT circuits cannot be fully integrated. For example, two connections to an integrated circuit are required for an external resonant circuit.
A further known solution consists of counting digitally the frequency to be tuned and to re-adjusting it accordingly. However, the expenditure on circuitry and the corresponding chip area in an integrated circuit required herefor are high. The current consumption for such a digital AFT circuit is also relatively high.